1. Field of Invention
This invention relates to Global Positioning (“GPS”) receivers, and in particular to multi-mode GPS receivers for use with wireless networks.
2. Related Art
The worldwide utilization of wireless devices such as two-way radios, portable televisions, Personal Digital Assistants (“PDAs”) cellular telephones (also generally known a “mobile phones” and/or “cell phones”), satellite radio receivers and Global Positioning Systems (“GPS”) is growing at a rapid pace. Cellular telephones, including Personal Communication System (“PCS”) devices, have become commonplace. The use of these wireless devices to provide voice, data, and other services, such as Internet access, has provided many conveniences to cellular system users. Additionally, the number of features offered by many wireless service providers is increasingly matching the features offered by traditional land-line telephone service providers. Features such as call waiting, call forwarding, caller identification (“caller I.D.”), three-way calling, data transmission and others are commonly offered by both land-line and wireless service providers. These features generally operate in the same manner on both wireless devices and land-line telephones.
Furthermore, other wireless communications systems, such as two-way paging, trunked radio, Specialized Mobile Radio (“SMR”) utilized by police, fire, and paramedic departments, have also become common mobile communications.
GPS systems (also known as Satellite Positioning System “SPS” or Navigation Satellite System) have also become commonplace. In general, GPS systems are typically satellite (also known as “space vehicle” or “SV”) based navigation systems. Examples of GPS systems include but are not limited to the United States (“U.S.”) Navy Navigation Satellite System (“NNSS”) (also know as TRANSIT), LORAN, Shoran, Decca, TACAN, NAVSTAR, the Russian counterpart to NAVSTAR known as the Global Navigation Satellite System (“GLONASS”) and any future Western European GPS such as the proposed “Galileo” program. As an example, the US NAVSTAR GPS system is described in GPS Theory and Practice, Fifth ed., revised edition by Hofmann-Wellenhof, Lichtenegger and Collins, Springer-Verlag Wien NewYork, 2001, which is fully incorporated herein by reference.
Typically, GPS receivers receive radio transmissions from satellite-based radio navigation systems and use those received transmissions to determine the location of the GPS receiver. It is appreciated by those skilled in the art that the location of the GPS receiver may be determined by applying the well-known concept of intersection utilizing the determined distances from the GPS receiver to three GPS satellites that have known GPS satellite locations.
Generally, each GPS satellite in a GPS satellite-based radio navigation system broadcasts a radio transmission, that contains its location information, and orbit information. More specifically as an example, each of the orbiting GPS satellites in the United States GPS system contains four highly accurate atomic clocks: two Cesium and two Rubidium. These clocks provide precision timing pulses, which are utilized in generating two unique binary codes (also known as a pseudo random noise “PRN,” or pseudo noise “PN” code), that are transmitted from the GPS satellites to Earth. These PN codes identify the specific GPS satellite in the GPS constellation.
Each GPS satellite also transmits a set of digitally coded ephemeris data that completely defines the precise orbit of the GPS satellite. The ephemeris data indicates where the GPS satellite is at any given time, and its location may be specified in terms of the GPS satellite ground track in precise latitude and longitude measurements. The information in the ephemeris data is coded and transmitted from the GPS satellite providing an accurate indication of the exact position of the GPS satellite above the earth at any given time.
Generally in GPS systems, there are four variables, namely, position determined by x, y, and z coordinates, and time (x, y, z, and t). These variables are determined by using triangulation techniques and accurate system clocks to determine the location of a GPS receiver through range, range-rate, and pseudo-range measurements made by or at the GPS receiver. To accurately determine the x, y, z, and t variables, four GPS satellite signals are typically needed to provide four simultaneous equations that are solved for the four variables.
These GPS satellites are configured, primarily, to provide a GPS receiver with the capability of determining its position, expressed for example by latitude, longitude, and elevation. This is typically accomplished by a resection process utilizing the distances measured from the GPS receiver to the GPS satellites.
As an example, if a GPS receiver utilized a clock that was precisely set to GPS system time a true distance, or range, to each GPS satellite from the GPS receiver could be accurately measured by recording the time required for the coded GPS satellite signal to reach the GPS receiver. Each range would define the surface of a sphere with its center at a given GPS satellite and the intersections of these spheres of at least three GPS satellites would yield three unknowns such as latitude, longitude, and elevation.
Unfortunately, GPS receivers typically utilize inexpensive crystal oscillator clocks that are set approximately to GPS system time. Therefore, these clocks is offset from the true GPS system time, and because of this offset, the distance measured to the GPS satellite differs from the “true” range. It is appreciated by those skilled in the art that these distances are known as “psuedoranges” because they are usually equal to the “true” range plus a range correction resulting from the GPS receiver clock error or bias. Generally, four simultaneously measured psuedoranges are needed to solve for four unknowns because these four unknowns include the three unknowns latitude, longitude, and elevation plus the GPS receiver clock bias (also known as a “time ambiguity”). It is appreciated that numerous well known techniques may be utilized to reduce the effects of the time ambiguity such as the solution described in U.S. Pat. No. 6,618,670, issued Sep. 9, 2003, and titled “Resolving Time Ambiguity in GPS Using Over-determined Navigation Solution,” which is herein incorporated by reference in its entirety.
With the growing widespread use of these technologies, current trends are calling for the incorporation of GPS services into a broad range of electronic devices and systems, including PDAs, cellular telephones, portable computers, radios, satellite radios, trucked radio, SMR, automobiles, two-way pagers and the like. At the same time, electronic device manufacturers constantly strive to reduce costs and produce the most cost-attractive product possible for consumers.
In cellular telephony, the interest of integrating GPS receivers with cellular telephones stems from a new Federal Communications Commission (“FCC”) requirement that cellular telephones be locatable within 50 feet once an emergency call, such as a “911” call (also referred to as “Enhanced 911” or “E911”) is placed by a given cellular telephone. When emergencies occur, people are accustom to dialing 911 (normally referred to as a “911” call) on a land-based (also known as “land-line”) telephone and contacting an emergency center that automatically is able to identify the location of the land-based telephone where the call originated.
Unfortunately, wireless devices, such as cellular telephones, are unable to communicate their location without a person actively entering or describing their location. In response, the United States Congress, through the FCC, has enacted a requirement that cellular telephones be locatable to within 50 feet once an emergency call, such as an E911, is placed by a given cellular telephone. This type of position data would assist police, paramedics, and other law enforcement and public service personnel, as well as other agencies that may need to have legal rights to determine the position of specific cellular telephone. The E911 services, however, operate differently on wireless devices than a 911 call does on land-line telephones.
When a 911 call is placed from a land-line telephone, the 911 reception center receives the call and determines the origin of the call. In case the caller fails, or forgets, to identify his or her location, the 911 reception center is able to obtain the location from which the call was made from the public telephone switching network (PSTN) and send emergency personnel to the location of the call.
If instead, an E911 call is placed from a wireless device such as a cellular telephone, the E911 reception center receives the call but cannot determine the origin of the call. If the caller fails, or forgets, to identify his or her location, the E911 reception center is unable to obtain the location of the call because the wireless network is different than the PSTN. At present, the best that the E911 reception center may do is to determine the location of the cell site from which the call was placed. Unfortunately, typical cell sites in a wireless network system may cover an area with approximately a 30-mile diameter. Further refinement of the location may be determinable in a digital network by the power setting of the calling wireless device. But, this still results in an area covering multiple miles.
A proposed solution to this problem includes integrating GPS receivers with cellular telephones. As an added benefit to this proposed solution is that any GPS data produced by an integrated GPS receiver may be utilized by the cellular telephone user for directions, latitude and longitude positions (locations or positions) of other locations or other cellular telephones that the cellular user is trying to locate, determination of relative location of the cellular user to other landmarks, directions for the cellular telephone user via internet maps or other GPS mapping techniques, etc. Such data may be of use for other than E911 calls, and would be very useful for cellular and PCS subscribers.
As an example of the current thrust to integrate GPS receivers with cellular telephony, U.S. Pat. No. 5,874,914, issued to Krasner, which is incorporated by reference herein, describes a method wherein a basestation (also known as a base station and/or the Mobile Telephone Switching Office “MTSO”) transmits GPS satellite information, including Doppler information, to a remote unit (such as cellular telephone) utilizing a cellular data link, and computing pseudoranges to the in-view GPS satellites without receiving or utilizing GPS satellite ephemeris information.
The approach in Krasner, however, is limited by the number of data links that can be connected to a GPS-dedicated data supply warehouse. The system hardware needs to be upgraded to manage the additional requirements of delivering GPS information to each of the cellular or PCS users that are requesting GPS data. These additional requirements would be layered on top of the requirements to handle the normal voice and data traffic that is managed and delivered by the wireless system.
Another patent that concerns assistance between the GPS system and wireless networks is U.S. Pat. No. 5,365,450, issued to Schuchman, et al. which is also herein incorporated by reference. In the Schuchman reference, ephemeris aiding through the cellular telephone system is required for the GPS receiver to acquire and track GPS satellites. However, cellular and other wireless networks do not always have the capability to provide ephemeris aiding to the mobile GPS receiver.
Therefore, there is a need in the art for delivering GPS data to wireless communications systems, including cellular and PCS subscribers, in an efficient manner. There is also a need for GPS capable cellular and PCS telephones. Moreover, there is a need for GPS capable cellular and PCS telephones that may receive GPS satellite data for use by the cellular/PCS subscriber (i.e., the user). Additionally, there is a need for a large cellular system that is capable of utilizing and/or supplying GPS information to cellular telephone users for a number of applications, including E911 without the requirement of geographically proximate basestations.